Predation on Artificial Nests in Small Grassland Patches in East-Central Illinois

Article excerpt

ABSTRACT.-We measured predation on artificial ground nests in 11 grassy fields ranging from 0.8-12.6 ha. We set out 12 nests in each field baited with two Japanese quail eggs at different distances (<10 m, 25 m, 50 m) from woody edges in early June, and another 12 nests baited with one quail and one zebra finch egg in each field in mid-July. Nests were monitored at 3-d intervals for 15 d. The proportion of nests in which quail eggs were depredated was 33% in June and 38% in July. Including predation on finch eggs raised the proportion of nests depredated to 78% in July, indicating that small-mouthed nest predators such as mice or shrews could account for a substantial amount of predation on nests of small passerine birds. Predation on nests was negatively related to area of field only for quail eggs in July (P = 0.04). We did not detect significant relationships between numbers of nests depredated or daily predation rates and distance to woody edges. The small size of fields in our study, which are typical of grassland patches in this region, may be below a threshold at which nests in the interior of patches experience reduced predation. Alternatively, highly fragmented natural habitats in an agricultural landscape may support high densities of predators, making edge and area effects difficult to detect.

INTRODUCTION

The effects of habitat fragmentation on avian communities in grasslands in the midwestern United States have received less attention than effects on avian communities in forests despite the fact that loss of grassland habitat has exceeded loss of forest habitat in this region (Herkert, 1994). Avian species associated with grasslands in this region are experiencing population declines that exceed those of most forest species (eg., Askins, 1993; Herkert, 1994, 1995). Of 27 grassland bird species examined by Herkert et al. (1995), 48% were considered endangered or threatened in at least one midwestern state. Avian species that nest in forest often suffer high rates of nest predation and parasitism in small forest fragments and near forest edges (eg., Andren, 1995; Faaborg et al., 1995; Robinson et al., 1995). Similar data for avian species nesting in small patches of grassland are increasing but still relatively few (e.g., Burger et at., 1994; Helzer and Jelinski, 1999; Winter and Faaborg, 1999). Additional information on how rates of nest predation in grasslands are affected by patch size or proximity to habitat edges could help conservation biologists and managers develop plans to conserve or restore grassland habitat for these species.

About 81% of the land area of Illinois is currently farmland (Neely and Heister, 1987) and 50% of the land area is covered by row crops (primarily corn, Zea mays, and soybeans, Glycine max). In east-central Illinois row-crop agriculture covers 75% of the land area (Mankin and Warner, 1992). Less than 20% of the state's remaining 245 native prairie remnants are >10 ha and less than 4% are >40 ha (Herkert et al., 1995). Other patches of grassland habitat such as old fields, hayfields and conservation reserve set-asides also can be small. Although many patches of grassland habitat are islands in a sea of row-crop agriculture, others are within complex mosaics of forest, wetlands, prairie restorations and successional fields. Area and edge effects on rates of nest predation in these remaining patches of grassland habitat could contribute to the observed declines of songbirds.

We used artificial ground nests (Wilcove, 1985) to compare rates of nest predation in grassland patches of various sizes, and at different distances from wooded edges, in a wildlife conservation area in east-central Illinois. Although fates of artificial nests may not be an accurate measure of the success of real nests (Willebrand and Marcstrom, 1988; Butler and Rotella, 1998), they provide a comparative index of predation rates that can identify habitats where nests may be more vulnerable to predation. …